FIRABLE MATERIAL FOR ELECTROSTATIC COATING

Abstract: A method of applying a glaze to a ceramic article, the method comprising applying a dry particulate glaze to the article by electrostatic application and retaining the glaze on the article prior to firing. The glaze is retained on the article before firing either by providing a thermoplastics material in the glaze which is melted on to the article, or by providing a moisture actuable material in the glaze.

Description: firable material for electrostatic coating This invention concerns a method of applying a layer of a firable material on to a firable article and particularly but not exclusively to applying glaze, and also an improved firable material and especially a glaze. Traditionally, glazing of ceramic tableware involves a wet process. Glaze is in the form of a water borne suspension of ground glaze, along with 5-10 % china clay suspending agent, and other additives such as binders, stabilisers, and colouring matter. The glaze is applied by a process of manual or semi-automatic dipping of ware, or by wet spraying of glaze onto ware. The process as a whole requires a high degree of monitoring as regards rheology, thixotropy, and fluidity of glaze 'slop' . Dipping and sprayed glazes obviously require rather different characteristics. Considerable effort is put into recovering spent glazes. For example, the overspray of the relatively inefficient wet glaze spray may be recovered using a waterfall. This recovery process necessarily involves the discharge of effluent waters which may require considerable and expensive water treatment to remove potentially toxic metals such as lead or zinc . The tableware is subsequently fired but before firing handleability of the tableware is required. This feature is necessary as a number of manual operations such as foot cleaning may be necessary or the ware may be stored for a short period such as a few days to fit into a firing programme before being moved into a kiln. It is essential therefore that no areas of glaze are rubbed or knocked off prior to firing. Attempts have been made to apply glaze to ceramic items by electrostatic dry powder application. In such application charged particles are usually sprayed onto an earthed article and are held thereon by the charge. However, this has not proved successful for tableware as it has been too easy to knock glaze off the ware or the applied powder has lost its charge and thus the glaze drops off prior to firing. Another problem which has been encountered when trying to use electrostatic spraying for ceramic ware is that due to the nature of the materials involved problems have been encountered in ensuring that the glaze is attracted to the ware. According to the present invention there is provided a method of applying a layer of a firable material to a firable article, the method comprising applying the material to the article by electrostatic application, and retaining the material on the article until subsequent firing of the material and article has t a k e n p l a c e . The material applied to the article is preferably particulate, and the material is preferably applied by electrostatic dry powder application, desirably by spraying . A material which can adhere to the article is preferably also applied onto the article to retain the firable material on the article. The adherable material is preferably of a type which will burn off during firing. The adherable material is preferably mixed with the firable material prior to applicati'on. The adherable material may adhere onto the article by solidifying thereon from a molten state. The article may be heated prior to application of the firable material such that the adherable material melts upon contact with the heated article, and solidifies thereon during subsequent cooling. Alternatively, the article and the materials thereon may be heated subsequent to application of the firable material to melt the adherable material, and then allowed to cool. The adherable material preferably comprises a thermoplastic material and may comprise polyethylene. The adherable material may be mixed with the firable material at elevated temperatures such that the adherable material melts during mixing. Alternatively, the adherable material may be moisture actuable. The adherable material may be applied dry onto the article and moisture subsequently introduced to the article, desirably by passing the article through a moisture chamber. The moisture actuable material may comprise a dextrin/water solution. Organic additives such as a modified cellulose may be provided in the solution. The resistivity of the firable material may be increased prior to application. The resistivity may be increased by mixing the firable material with a highly resistive material which may comprise a liquid curable siloxane . The highly resistive material is preferably of a type which will burn off during firing. Alternatively, the highly resistive material may leave a residue after firing . An electrically conductive layer may be applied to the article prior to electrostatic application. The conductive layer may comprise an aqueous solution of a salt. The present invention also provides a method of applying glaze to a ceramic article using a method according to any of the preceding sixteen paragraphs. The invention still further provides a particulate firable material which can be electrostatically applied onto a firable article and handled without any substantial damage thereto prior to subsequent firing. The firable material preferably includes a material which adheres to the article. The adherable material is preferably of a type which will burn off during firing. The adherable material may be of a type which adheres by solidifying from a molten state. The adherable material preferably comprises a thermoplastic material and may comprise polyethylene. The firable material preferably comprises 1 to 2D?ό thermoplastic material by weight, and desirably 10 to 1 ?ό. Alternatively the adherable material may be moisture actuable. The moisture actuable material may comprise a dextrin/water solution. Organic additives such as a modified cellulose may be provided 'i-n the solution. The firable material preferably comprises 1 to 20 % dextrin by weight, and desirably 5 to 10 % . The firable material may also comprise a highly resistive material which may comprise a liquid curable siloxane or a liquid glycol such as ethylene glycol. Where present, the siloxane preferably comprises 0.01 to 0.1 % siloxane by weight of firable material. Where present the liquid glycol preferably comprises up to 5?ό by weight of the firable material. The highly resistive material is preferably of a type which will burn off during firing. Alternatively, the highly resistive material may be of a type which will leave a residue after firing. The firable material may comprise an antiblocking agent, which may be fumed silica. The invention yet further provides a firable material in the form of a glaze according to any of the preceding ten paragraphs which can be applied by electrostatic application. Embodiments of the present invention will now be described by way of example only. The preparation of a glaze powder according to the present invention will now be described. 1 kg of dry Blythe 72C5 leadless glaze frit and 5 g of Wacker Chpmicals' Polymethyl Hydrogen siloxane (PMHS) were placed in a ball mill with 3 kg mixed alumina grinding media. The mill was then placed on rollers set to turn the mill at 120 rpm for 9 hours. The resulting powder was then separated from the grinding media and placed in a drying oven at 200°C for 1 hour to cure the PMHS. The powder was then cooled and sieved through a 200 micron aperture mesh and any particles retained on the sieve were discarded. The remaining powder was weighed and placed in a small Z-Blade mixer which was then heated to 160°C. A 15SK by weight addition of 70% BDH 'Paraffin Wax' (98°C) and 30 % shell low density polyethylene was calculated and weighed out. This addition was then placed into the mixer and the blades switched on. After 5 minutes mixing the temperature was turned down to 140°C and mixing continued. After a further 20 minutes the mixture had formed a single 'doughlike' mass. At this point the blades were switched off and the mixture cooled to 80°C. Once cooled the blades were switched on and the mixture 'self granulated' . When the mixture ceased to granulate further it was removed from the mixer and allowed to cool to room temperature. The granulated mixture was sieved through a 4 mm mesh sieve and any pieces retained broken down by hand in a mortar and pestle till all passed. The granulated mixture was then loaded into a Christison Mixer Mill 2000 and ground for 5 minutes. The ground mixture was then silica was made. This was mixed in and the powder sieved through a 200 micron mesh with any particles retained being reground until all passed. The resulting powder was weighed and a 0.2°ό by weight addition of fumed resieved through the 200 micron mesh to evenly distribute the fumed silica. The glazed powder thus formed can be stored until required and then applied to an item of tableware by electrostatic spraying. In this process a large potential difference of typically 60 to 100 kV is set up between the tip of an electrostatic gun and the workpiece. The glaze powder passes through this zone fed by low pressure air and becomes charged, typically by picking up electrons. The sprayed material is attracted to the earthed workpiece where it settles and is held by induced electrostatic charge, or by the action of the adherable polyethylene, or both. In one arrangement the ware may be heated such that the thermoplastic (polyethylene) melts on the ware and upon subsequent cooling adheres to the ware holding the glazed material thereon until firing. This permits handling of the ware for manual operation such as foot cleaning and also prevents the glaze from being knocked off the ware during for example loading onto kiln furniture. In an alternative arrangement the ware may be at room temperature during spraying and may subsequently be heated and allowed to cool to cause the thermoplastic to melt and subsequently adhere thereto. The siloxane mixed with the glaze increases the electrical resistivity thereof permitting good electrostatic application onto the ware. Without this increased resistivity the usual low resistivity of the glaze which is caused by the mobile cations on the glaze particle surface (sodium, calcium, potassium etc.) rapidly disperse the applied charge. This would make for very poor electrostatic spraying of the ware and also a high possibility of any glaze which was attracted to the ware falling off before being held thereon by the thermoplastic. The thermoplastic is of course burnt off during the firing. The siloxane adsorbs onto the glaze particles thereby producing an at least partial coating. The siloxane helps the thermoplastic to key on to the glaze particles . Usual constituents for glaze powders including a thermoplastic material are as follows: Glaze frit - siloxane added Frit 99.9 to 99.99% ) By Siloxane 0.01 to 0.1% ) Weight Glaze/frit - siloxane added 85-90% ) By Thermoplastic Material 10-15% ) Weight Thermoplastic Material 65-80% Synthetic polyethylene 'Paraffin Wax' . Melting point 90-100°C 20-35% Low density poly¬ ethylene. Melting point = 130°C An alternative to the thermoplastic is a moisture actuable adhesive such as dextrin. A typical example of a glaze powder including dextrin is as follows: Binder and frit formula Coated frit 90 to 95% ) By Technical Grade Yellow Dextrin 5 to 10% ) Weight Organic additives such as a modified cellulose may be added. These materials are mixed at high shear in a 2% solution of Polyvinyl alcohol (88% hydrolysed molecular weight 25,000 typical). The ratio of solid to PVA solution being 1:1 to 1:0.8. The material is carefully dried out at 40-80 D C in a hot air oven with occasional mixing to distribute the dextrin. The above materials can be dry ground together using a liquid glycol, typically ethylene glycol. The ratio of glaze to ethylene glycol to modified cellulose is 1 to 0.1 to 0.02. Such a glaze would normally be applied dry and the coated ware is then passed through a humidity chamber for typically a 1 to 2 minute period to activate the dextrin adhesive. The dextrin imparts excellent handleability to the ware and is subsequently burnt off during firing. There is thus described a method of applying glaze to tableware, and a glaze to be applied in this manner, which provide considerable advantages. The invention permits dry electrostatic spraying to be used for glaze application. This type of spraying has three main advantages. The first of these is wrap around i.e. that the charged powder coats the ware from all angles and not just from the direction it is sprayed. Secondly in certain conditions the coating formed is self-limiting in that the charge on the powder builds up as the powder layer increases until a point is reached where further incoming powder is repelled from the workpiece. Thirdly, electrostatic spraying also provides a very even layer, and a very smooth surface appearance is achievable if required subsequent to firing. As well as providing increased resistivity to aid electrostatic attraction the coating renders the glaze powders hydrophobic. This has an additional bonus in the second step, the mixing in the Z-Blade mixer, of preparation of glaze powders. Different ceramic bodies have different conductivities and thus require different glaze compositions and spraying conditions. For bodies which have low conductivity e.g. bone china bisque, it is important to use the thermoplastic and to heat the ware such that the thermoplastic provides adhesion onto the ware at an early moment. In view of the low conductivity of some vitreous bodies it may be necessary to firstly apply a conductive layer onto the ware's surface such as an aqueous solution of a soluble salt. In contrast, clay ware is an ideal substrate for electrostatic application. Its relatively high moisture content renders the material sufficiently conductive for electrostatic dry glaze application. Clay ware is particularly suited for glazing with the dextrin glaze. Additional benefits of the thermoplastic system are the excellent handleability which can give protection to delicate underglaze line decorations. Furthermore, such a glaze can be used to cover unfired underglazed decorations. These generally include an organic binder which tends to cause rejection with conventional water based glaze systems. This is not a problem with the dry thermoplastic coated glazes. Both the thermoplastic and dextrin glazes burn out cleanly giving combustion products of carbon dioxide and water. Such glazes are of low allergenic character and are generally non-toxic. The method of glaze application according to the present invention is environmentally much more friendly than conventional methods because the overspray being a powder is more easily recove ' red and is almost totally recovered. Accordingly, the problems with reclaiming spent glaze and also avoiding toxic effluent are avoided . Various modifications may be made without departing from the scope of the invention. For example different materials may be used other than those outlined above. Material other than siloxane, or a liquid glycol could be used to increase resistivity. Different adhesive materials could be used. Furthermore, different processes may be used to prepare the dry glaze powders. It is to be realised that the present invention is usable for applying materials to firable articles other than glazes to tableware. For example, the invention may be used to apply frits or colours to a wide range of materials. Furthermore, layers can be applied to electrical and electronic components or a wide range of other firable substrates such as glass. Whilst endeavouring in the foregoing specification to draw attention to those features of the invention believed to be of particular importance it should be understood that the Applicant claims protection in respect of any patentable feature or combination of features hereinbefore referred to and/or shown in the drawings whether or not particular emphasis has been placed thereon.

Claims: C l a ims : - 1. A method of applying a layer of a firable material to a firable article, the method comprising applying the material to the article by electrostatic application, characterised in that the material is retained on the article until subsequent firing of the material and article has taken place. 2. A method according to Claim 1, characterised in that the material applied to the article is particulate. 3. A method according to Claim 2, characterised in that the firable material applied is applied by electrostatic dry powder application. 4. A method according to Claim 3, characterised in that the firable material is applied by spraying. 5. A method according to any of the preceding claims, characterised in that a material which can adhere to the article is also applied onto the article to retain the firable material on the article. 6. A method according to Claim 5, characterised in that the adherable material is of a type which will burn off during firing. 7. A method according to Claims 5 or 6, characterised in that the adherable material is mixed with the firable material prior to application. 8. A method according to any of Claims 5 to 7, characterised in that the adherable material adheres onto the article by solidifying thereon from a molten state . 9. A method according to Claim 8, characterised in that the article is heated prior to application of the firable material such that the adherable material melts upon contact with the heated article, and solidifies thereon during subsequent cooling. 10. A method according to Claim 8, characterised in that the article and the materials thereon are heated subsequent to application of the firable material to melt the adherable material, and then allowed to cool. 11. A method according to any of Claims 8 to 10, characterised in that the adherable material comprises a thermoplastic material. 12. A method according to Claim 11, characterised in that the adherable material comprises polyethylene. 13. A method according to any of Claims 8 to 12, characterised in that the adherable material is mixed with the firable material at elevated temperatures such that the adherable material melts during mixing. 14. A method according to any of Claims 5 to 7, characterised in that the adherable material is moisture actuable . 15. A method according to Claim 14, characterised in that the adherable material is applied dry onto the article and moisture subsequently introduced to the article . 16. A method according to Claim 15, characterised in that moisture is subsequently introduced to the article by passing the article through a moisture chamber. 17. A method according to any of Claims 14 to 16, characterised in that the moisture actuable material comprises a dextrin/water solution. 18. A method according to Claim 17, characterised in that organic additives are provided in the solution. 19. A method according to Claim 18, characterised in that the organic additives comprise a modified cellulose . 20. A method according to any of the preceding claims, characterised in that the resistivity of the firable material is increased prior to application. 21. A method according to Claim 20, characterised in that the resistivity is increased by mixing the firable material with a highly resistive material. 22. A method according to Claim 21, characterised in that the highly resistive material comprises a liquid curable siloxane. 23. A method according to Claims 21 or 22, characterised in that the highly resistive material is of a type which will burn off during firing. 24. A method according to Claims 21 or 22, characterised in that the highly resistive material leaves a residue after firing. 25. A method according to any of the preceding claims, characterised in that an electrically conductive layer is applied to the article prior to electrostatic application . 26. A method according to Claim 25, characterised in that the conductive later comprises an aqueous solution of a salt. 27. A method of applying glaze to a ceramic article characterised in that a method is used according to any of Claims 1 to 26. 28. A particulate firable material characterised in that the material can be electrostatically applied onto a firable article and handled without any substantial damage thereto prior to subsequent firing. 29. A material according to Claim 28, characterised in that the firable material includes a material which adheres to the article. 30. A material according to Claim 29, characterised in that the adherable material is of a type which will burn off during firing. 31. A material according to Claims 28 or 29, characterised in that the adherable material is of a type which adheres by solidifying from a molten state. 32. A material according to Claim 31, characterised in that the adherable material comprises a thermoplastic ma t e r i a l . 33. A material according to Claim 32, characterised in that the adherable material comprises polyethylene. 34. A material according to Claims 32 or 33, characterised in that the firable material comprises 1 to 20% thermoplastic material by weight. 35. A material according to Claim 34, characterised in that the firable material comprises 10 to 15% thermoplastic material by weight. 36. A material according to Claims 28 or 29, characterised in that the adherable material is moisture actuable . 37. A material according to Claim 36, characterised in that the moisture actuable material comprises a dextrin/water solution. 38. A material according to Claims 36 or 37, characterised in that organic additives are provided in the solution. 39. A material according to Claim 38, characterised in that the organic additives comprise a modified cellulose . 40. A material according to any of Claims 36 to 39, characterised in that the firable material comprises 1 to 20% dextrin by weight. 41. A material according to Claim 40, characterised in that the firable material comprises 5 to 10% dextrin by weight . 42. A material according to any of Claims 28 to 41, characterised in that the firable material comprises a highly resistive material. 43. A material according to Claim 42, characterised in that the resistive material comprises a liquid curable siloxane. 44. A material according to Claim 43, characterised in that the siloxane comprises 0.01 to 0.1% by weight of the firable material. 45. A material according to Claim 42, characterised in that the resistive material comprises a liquid glycol. 46. A material according to Claim 45, characterised in that the resistive material comprises ethylene glycol. 47. A material according to Claims 45 or 46, characterised in that the liquid glycol comprises up to 5% by weight of the firable material. 48. A material according to any of Claims 42 to 47, characterised in that the highly resistive material is of a type which will burn off during firing. 49. A material according to any of Claims 42 to 47, characterised in that the highly resistive material is of a type which will leave a residue after firing. 50. A material according to any of Claims 28 to 49, characterised in that the firable material comprises an antiblocking agent. 51. A material according to Claim 50, characterised in that the antiblocking agent comprises fumed silica. 52. A firable material in the form of a glaze which can be applied by electrostatic application, characterised in that the material is according to any of Claims 28 to 51.

Inventors: BRETT ROBERT DENNIS, CROOKS MARK EDWARD, SHINGLER DAVID, WITHINGTON STEVEN CHARLES, BRETT, ROBERT, DENNIS, CROOKS, MARK, EDWARD, SHINGLER, DAVID, WITHINGTON, STEVEN, CHARLES
Applicants: BRETT ROBERT DENNIS, BRITISH CERAMIC RES LTD, CROOKS MARK EDWARD, SHINGLER DAVID, WITHINGTON STEVEN CHARLES, BRETT, ROBERT, DENNIS, BRITISH CERAMIC RESEARCH LIMITED, CROOKS, MARK, EDWARD, SHINGLER, DAVID, WITHINGTON, STEVEN, CHARLES
Application year: 1994
Main classification: C04B41
IPC codes: C03C17, C04B41, C23D5